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Ch 18: Thermal Properties of Matter
Young & Freedman Calc - University Physics 14th Edition
Young & Freedman Calc14th EditionUniversity PhysicsISBN: 9780321973610Non è quello che usi tu?Cambia libro di testo
Capitolo 18, Problema 36a

Smoke particles in the air typically have masses of the order of 10−1610^{-16} kg. The Brownian motion (rapid, irregular movement) of these particles, resulting from collisions with air molecules, can be observed with a microscope. Find the root-mean-square speed of Brownian motion for a particle with a mass of 3.00×10−163.00\(\times\)10^{-16} kg in air at 300300 K.

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Start by understanding the concept of root-mean-square speed, which is a measure of the average speed of particles in a gas. It is derived from the kinetic theory of gases.
Use the formula for root-mean-square speed: 3kTm, where k is the Boltzmann constant (1.38×10-23J·K-1), T is the temperature in Kelvin, and m is the mass of the particle.
Substitute the given values into the formula: 3×1.38×10-23·3003.00×10-16.
Calculate the numerator: 3×1.38×10-23·300, which represents the product of three constants and the temperature.
Calculate the root-mean-square speed by taking the square root of the fraction obtained from the previous steps, which involves dividing the calculated numerator by the mass of the particle.

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Brownian Motion

Brownian motion refers to the random movement of particles suspended in a fluid, resulting from collisions with fast-moving molecules in the fluid. This phenomenon is crucial for understanding the behavior of smoke particles in air, as it explains their rapid and irregular motion observed under a microscope.
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Root-Mean-Square Speed

The root-mean-square speed is a measure of the average speed of particles in a gas, derived from the kinetic theory of gases. It is calculated using the formula v_rms = sqrt((3kT)/m), where k is the Boltzmann constant, T is the temperature, and m is the mass of the particle. This concept helps quantify the speed of Brownian motion for smoke particles.
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Root-Mean-Square Speed of Ideal Gases

Kinetic Theory of Gases

The kinetic theory of gases explains the macroscopic properties of gases by considering their molecular composition and motion. It provides the basis for calculating the root-mean-square speed, as it relates temperature and particle mass to the average kinetic energy, essential for understanding the dynamics of particles like those in Brownian motion.
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Introduction to Kinetic-Molecular Theory
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Calculate the mean free path of air molecules at 3.50×10−133.50\(\times\)10^{-13} atm and 300300 K. (This pressure is readily attainable in the laboratory; see Exercise 18.2318.23.) As in Example 18.818.8, model the air molecules as spheres of radius 2.0×10−102.0\(\times\)10^{-10} m.

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Compute the specific heat at constant volume of nitrogen (N2) gas, and compare it with the specific heat of liquid water. The molar mass of N2 is 28.028.0 g/mol.

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At what temperature is the root-mean-square speed of nitrogen molecules equal to the root-mean-square speed of hydrogen molecules at 20.020.0°C? (Hint: Appendix D shows the molar mass (in g/mol) of each element under the chemical symbol for that element. The molar mass of H2 is twice the molar mass of hydrogen atoms, and similarly for N2.)

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